This invention relates to improvements in systems and methods for the control of the fuel supply for diesel engines, and particularly to such control systems which are primarily electrical in nature. Electrical control systems of this class are commonly known as electrical governors, and in the preferred embodiment of the invention described herein in which the principal element of the governor is a microprocessor, are commonly designated as microprocessor governors.
More particularly, the invention concerns itself in one primary aspect with the prevention or minimizing of puffs of smoke which tend to be produced transiently when the fuel to a turbocharged diesel engine is suddenly and rapidly increased. Such puffs of smoke, although generally appearing only momentarily, are nevertheless objectionable because of the high visibility of their rather slight contamination of the environment, and to some extent because they may be seen as indicative of some wastage of fuel. Further, the intensity of the smoke, i.e. its visual opacity is limited by law.
The type of engine system with regard to which the invention will be specifically described, and to which it is particularly applicable, comprises a turbocharged diesel engine for operating a load, a fuel injection pump for delivering timed, metered quantities of diesel fuel to the cylinders of the engine in timed succession, an operator-controlled throttle control, such as a foot throttle, and an electrical governor which controls the position of the fuel control rack in the fuel injection pump to determine the quantity of fuel delivered to each engine cylinder during each cycle of the engine. An engine system of this type is, for example, particularly suited for use in automotive vehicles such as large trucks.
In the past, mechanical governors have commonly been used to control the supply of fuel to diesel engines. In such use the governor is commonly known as a variable speed governor in that it permits wide variation of the speed of the engine in response to operation of the operator's control throttle. At the same time, the governor typically also provides a number of limits on the rate at which fuel can be delivered to the engine at various engine speeds. One such typical limit is provided by the so-called low-idle control, which prevents the engine speed from falling below a level which might produce stalling of the engine; there is typically also employed a high-speed limit which limits the fuel delivery rate if the engine speed tends to exceed a predetermined maximum value, to protect the engine from the harm which would result from excessively high-speed operation; further, within the speed range between the low-idle limit and the high-speed limit, there is typically provided a so-called torque limit of fuel which prevents over-fueling of the engine in the intermediate range of engine speeds. The construction and the operation of such mechanical governors are well known and have been presented fully in patents and other technical publications.
In such mechanically governed engine systems using a turbocharger for the engine, the above-described objectionable phenomenon of smoke puffs also existed unless special measures were taken to prevent it. These puffs occur when the air charge supplied to the cylinders of the diesel engine is insufficient to accomplish complete combustion of the amount of fuel injected into the engine cylinders at that time. In a turbocharged engine, under steady-state operating conditions quite high injected fuel levels can be used without smoke production because the turbocharger injects compressed air into the cylinders; the compressed air contains a higher concentration of oxygen molecules than ordinary air at atmospheric pressure, and is therefore able to provide for the desired complete combustion of the fuel under steady-state conditions. The air compressor of the turbocharger is typically driven directly from a small turbine in the engine exhaust line, and the speed of the turbine increases with the kinetic energy of the exhaust gas flow and hence with the load on the engine and, to some extent, with increases in the speed of the engine. The turbine typically drives the air compressor in the turbocharger directly in such manner that, under steady-state operating conditions of the engine, the large amount of fuel delivered to the engine under full load conditions is combusted sufficiently fully to avoid objectionable smoking; this is because the exhaust gases are then of high kinetic energy, operate the turbocharger turbine at a high rate, and cause the compressor in the turbocharger to produce air sufficiently pressurized to provide the desired complete combustion. Thus, despite high levels of fuel delivered to the engine, the turbocharger automatically delivers enough air to the engine to assure the desired lean mixture for which objectionable smoking does not occur.
If the mechanical governor and the turbocharger could respond and act entirely instantaneously, there would always be sufficient air to completely burn the fuel in the engine, and smoking would never occur. Unfortunately, the system is not capable of such instantaneous action, and there is always some lag between the time when a sudden increase in fuel occurs and the time when the air charge to the engine cylinders has been increased to the level required to burn the increased amount of fuel completely. Accordingly, while the air charge is "catching up" to the fuel increase, there will be a momentary deficiency of air with resultant smoke. Such air deficiencies and smoke can occur when the operator control calls for a sudden increase in fuel in order to accelerate the vehicle from a low speed. It also typically occur during shifting of gears, at which time declutching of the engine from the load throws the engine into a no-load condition at the same time that the operator permits the engine speed to decrease; this greatly reduces the energy of the exhaust gases, slows down the turbine, slows down the compressor, and greatly reduces the air charge into the engine. When the operator reengages the clutch and applies power through his throttle control, the resultant large and sudden increase in fuel to the engine is not immediately accompanied by sufficient delivery of air charge to the engine, and objectionable smoke momentarily appears from the exhaust until the increased exhaust energy can bring the turbocharger up to the speed required for complete fuel combustion.
Puff control devices have been proposed for use in mechanical governors to eliminate or minimize the generation of smoke puffs described above. Without describing the make-up of such mechanical governors and their puff control devices in detail, it is sufficient for the present purposes to mention that such mechanical governors typically include a torque plate which limits the extent to which the fuel pump rack can move in the fuel-increasing direction under full load conditions. To minimize the production of smoke puffs, it is possible to mount the torque plate for reciprocating motion, spring bias it in the direction which results in reduced fuel delivery, and provide a diaphragm and piston arrangement connected to the torque-plate supporting shaft and supplied on one side with pressurized air at the same pressure as that supplied to the engine intake manifold from the turbocharger. The arrangement is such that when the the manifold air pressure is high the torque plate will be moved against the spring bias to permit a high level of maximum fuel delivery; however, should the manifold air pressure be unusually low when the fuel delivery rate is high, the spring will advance the torque plate in the fuel-decreasing direction and limit the fuel delivery to a level which will avoid generating smoke puffs.
The latter mechanical system has at least several limitations. The primary problem is that the system generally does not hold the maximum fuel rate just below the smoke level while the turbocharger is accelerating during the puff generating interval, as would be desired for maximum efficiency. That is, the movement of the torque plate does not accurately track the pressure-recovery curve of the turbocharger. Typically, the torque plate will stay at its steady-state position without substantial movement by the puff control system until a very substantial deficiency of turbocharger air pressure has accumulated, and then will move quite rapidly to, or very nearly to, its extreme low fuel position. Once there, it tends to remain there until the turbocharger has recovered speed sufficiently to produce a very substantial air pressure increase, and then moves rapidly to, or close to, its maximum fuel position again. As a result, one can design such system in either of two general ways, neither of which is completely satisfactory; it can be designed so that the torque plate moves readily and easily between its two extreme positions in response to small pressure changes, in which case it will permit reapplication of high fuel delivery rates too quickly, with resultant undesirably high smoke levels in the puff; or, it can be made so that the torque plate is relatively hard to move so that such undesirably high levels of smoke in the puff are prevented, but the time required for the fuel to resume its higher steady-state rate will then be unduly prolonged.
In recent years it has been proposed that the fuel, air and even the timing of a turbocharged diesel engine be controlled electrically rather than mechanically. In such an arrangement, various significant parameters of engine operation are sensed, converted to electrical signals, and supplied to an electrical control system which operates on the received sensor signals to produce output control signals for controlling the major engine operating parameters. While in general this can be done using discrete analog electrical components, it is presently preferred to do this by means of digital circuitry, preferably greatly miniaturized, as in a so-called microprocessor. The microprocessor normally comprises small integrated-circuit semi-conductor chips especially designed for the particular application, and can be made very small and can be mounted at any convenient location adjacent the engine. Typically, in such a system a sensor may be provided to produce an electrical signal indicative of the operator's instantaneous setting of the throttle control by which he indicates the level of engine fuel which he wishes to produce, as by sensing the position of the operator's foot-throttle linkage. Electrical signals indicative of engine speed are readily obtained by providing a small magnetic sensing or pick-up device positioned adjacent the teeth of a gear rotating with the engine drive shaft, and counting the number of pulses generated per second by conventional electronic means. In such case the usual arrangement of mechanical governor with fly weights, adjustable fulcrum lever and torque-limiting stop plate may be entirely eliminated, and the position of the rack which controls the rate of fuel delivery by the fuel injection pump controlled entirely by the electrical control system, which may then be termed an electrical governor since it supplies functions of the earlier mechanical governor.
In such an electrical governor system it would be possible to sense the engine intake manifold pressure produced by the turbocharger by means of an appropriate pressure-sensing device in the manifold. The pressure thereby sensed could be converted to an electrical signal and supplied to the electrical governor system, and the latter pressure-representing signal could then be used to limit the amount of fuel supplied to the engine to a level which does not produce objectionable smoke.
A drawback of the latter arrangement lies in the cost of providing an appropriate sensor which would operate with accuracy in the environment of the engine intake manifold, and mounted so as to be easily replaceable. Further, even a relatively costly sensor can be expected to fail prematurely on occasions, and even if it does not do so it can be expected to have some limited life, ultimately requiring the cost of a replacement sensor. In addition, it can be expected that such a sensor may on occasions deteriorate gradually over a period of time during which it is not entirely inoperative but instead provides inaccurate information as to pressure, a situation which may be difficult to diagnose from overall engine operation, and will generally require a decision as to when it has deteriorated to the point where it should be replaced, leaving the possibility of unsatisfactory operation over a period of time with excessive generation of objectionable puffs until replacement is finally made.